1. Field of the Invention
The present invention relates to halogen-containing polycarbonate resins which are non-inflammable or inflammable only with difficulty and to an industrially advantageous process for preparing the same using phosgene.
2. The Prior Art
Various methods are known for the production of a halogen-containing polycarbonate resin by a phosgene process, for example:
(1) U.S. Pat. No. 3,062,781 discloses a method in which a mixture of a halogenated bisphenol A and bisphenol A is reacted with phosgene to directly yield the resin; PA1 (2) Japanese Patent publication No. 7147, 1963 discloses a method in which bisphenol A is reacted with phosgene to produce a polychloroformate (i.e. a polycarbonate oligomer), which is then reacted with a halogenated besphenol A and bisphenol A to obtain a resin of high molecular weight; and PA1 (3) a method wherein (1) polycarbonate prepared from bisphenol A and (2) a polycarbonate prepared from tetrachlorobisphenol A are catalytically polymerized. PA1 (1) co-polymerizing the following oligomers (a) and (b) in the presence of caustic alkali an organic solvent, water and in the substantial absence of residual bisphenol A and in the absence of added bisphenol A to form a solution of the halogen-containing polycarbonate in the organic solvent; PA1 (2) washing the organic solvent solution containing the halogen-containing polycarbonate resin with an aqueous washing agent; and PA1 (3) recovering the polycarbonate resin from the organic solvent.
In the production of a polycarbonate resin from bisphenol A by means of the phosgene process, the resin is usually purified by washing the organic solvent solution of the resin, obtained by the polymerization reaction, with an aqueous washing agent such as water or an acid solution. After the washing for purification ("refining"), the organic solvent solution of the resin is concentrated to cause the resin to precipitate from solution in the form of a powder. If an attempt is made to produce a polycarbonate resin containing a relatively large amount of halogen, e.g. the 4% by weight required to render the resin essentially flameproof, using the conventional methods, it is extremely difficult to wash and concentrate the resin/solvent solution obtained by the polymerization and to pulverize the resin using the conventional methods, as compared with the case wherein only bisphenol A is used as the starting material. Moreover, polycarbonate resins of high halogen content obtained by the conventional methods have inferior melt fluidity as compared with a polycarbonate resin prepared from bisphenol A alone.
For example, a methylene chloride solution containing 10% by weight of a polycarbonate resin with a bromine content of 4% by weight, prepared by the above-mentioned method (2), i.e. by the reaction of a polycarbonate oligomer with tetrabromobisphenol A and bisphenol A, forms an emulsion when subjected to washing on an industrial scale, thus hindering the separation of the organic solvent phase containing the resin from the aqueous phase, or, if the separation is possible, numerous dirt layers form which adversely affect the washing efficiency and which make adequate removal of the impurities difficult. Accordingly, the resin product tends to have inferior color and low thermal stability. Further, the resin product is recovered as a viscous gel which is very difficult to pulverize and which inhibits production on an industrial scale.
Accordingly, it is an object of the present invention to provide a flameproof polycarbonate resin having good color, high thermal stability and good melt fluidity.
It is a further object to provide such a polycarbonate resin having a high halogen content.
Yet another object of the invention is to provide an industrially advantageous process for production of such a resin wherein washing, concentrating and pulverizing steps are facilitated.
Other objects and features of the present invention will become apparent from the detailed description to follow.